Complex numbers

I am trying to solve this set of problems, any ideas:

Let 1, omega, omega squared, ....., omega^n-1 be nth rooots of unity
then
(a) Show the conjugate of any nth root of unity is another root of unity, by expressing omega(bar)^j in the form omega^k for appropriate k.
(b) Find the product of the nth roots of unity
(c) Find the sum Summation (n-1), k=0 for omega^k of nth roots of unity. (this is a geometric series).